bundle/statemgmt: add StateReader abstraction for file and DMS state

[shreyas-goenka]

Summary

Adds a self-contained StateReader abstraction for reading the direct engine's
resource state, with two implementations:

type StateReader interface {
    Load(ctx context.Context, db *dstate.DeploymentState) error
}

• NewFileStateReader(path) — reads local resources.json (delegates to
  DeploymentState.Open, so WAL recovery + migration are preserved).
• NewDMSStateReader(client, deploymentID, path) — reads from the deployment
  metadata service via Bundle.ListResourcesAll, mapping each DMS resource into
  a state entry (prefixing keys with resources., copying resource_id +
  serialized state).

This is deliberately not wired into the deploy path. It is a drop-in seam to
replace the current StateDB.Open call in process.go once the DMS lock and
op-reporting PRs land; selecting the reader by managed-state and sourcing the
deployment ID are integration-time concerns that depend on those PRs.

Rework notes (vs the previous version of this PR)

The earlier version inlined LoadStateFromDMS and scattered managed-state
branches across state_pull.go / state_push.go / process.go, stacked on the
now-superseded old DMS-lock branch. This version instead:

• Models the two backends as one interface with two implementations, matching
  the house style of lock.DeploymentManager and direct.opRecorder.
• Keeps the file path delegating to DeploymentState.Open so WAL recovery and
  state migration are not lost.
• Is based on main and carries no integration changes, so it can land
  independently and be integrated later.

Test plan

• go test ./bundle/statemgmt/... (new TestDMSStateReader* / TestFileStateReader*)
• go build ./...
• go tool golangci-lint run ./bundle/statemgmt/ — 0 issues

[eng-dev-ecosystem-bot]

Commit: 71a41d9

Run: 26727951172